1. Field of the Invention
The present invention relates to a method of and apparatus for preprocessing and vacuum-frying vegetables and fruits for preparing desicated foods, etc.
2. Description of the Prior Art
A vacuum fry treatment has widely been employed as a method of processing desicated foods, for which many inventions and devices have been applied. In general, vacuum frying processing supplies heat to a raw material by making use of fats and fatty oils, which are excellent in heat transfer efficiency and dries up and dehydrates the material for a short period of the time. In addition, a food treatment under reduced pressure assures a frying process, whereby deterioration of quality of the material such as browning and fading of the material can be prevented. Moreover, processing at low temperature and low pressure reduces deterioration and oxidation of frying oil and assures a reduced content of the oil penetrating into the material as well as a product excellent in shelf-life stability.
However, such vacuum frying processing violently evaporates 70 to 80% of the water in the material 3 to 5 minutes after starting the processing, whereby the material deforms and contracts and the oil temperature is appreciably lowered to decrease a drying rate. As the result, with an amount of preparing a raw material at once being increased, more time is needed to restore the original high oil temperature, and thus, the frying time is very unstable with the result of providing products having variable quality. To solve the drawbacks described above, primary drying treatments of various kinds to be effected prior to vacuum frying processing have been devised to perform the vacuum frying processing for a short period of time and improve productivity. However, these treatments require separate dryer equipment which may take a lot of time for drying the material and the quality of the material is deteriorated because of the processing at high temperature or a color tone is changed.
Moreover, almost all of the apparatuses to achieve the primary drying treatments are of a batch type of a single frying tank or a continuous type which use a conveyer, and which are not only difficult to keep at a high vacuum, but also produce a bad product because chips of the raw material are softened and deposited on the upper part of a fry basket upon violent evaporation at the beginning of the frying processing. Furthermore, the frying oil becomes dirty and deteriorates the color tone of the product as well as make its taste bad.